I'm afraid I forget, but it wasn't very early. Apparently it is sometimes possible from as early as week 12:
but I'd think that you would need to be very experienced to realise what you were hearing, as it is barely perceptible when you first try it.
I had the benefit of our retired-gynecologist aunt confirming that what I thought I was hearing was actually real.
You could do worse that going to your gynecologist with a spare print, and presenting it to them in exchange for a quick lesson -- ours seemed very pleased with the present :-)